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ThinkPad X220 with a 9-cell, plus an additional battery pack for when you're travelling.

Both battery slice and 9 cell battery exceeds the desired weight.

With 6 cells you get somewhere around 7-8 hours if you are lucky. I know this laptop, but looking for better options.

You will get more than 8 hours with just the 9 cell, which will bring it to under your weight - http://forum.thinkpads.com/viewtopic.php?f=43&t=98466

You don't need the 9 cell as well as the plate - it was just a suggestion if you needed more.